RFID technology concerns identification techniques enabling remote data retrieval via radio frequencies from RFID devices hereinafter also referred to as “tags”. The tags can be included in or attached to target objects such as different kinds of products or humans/animals that are to be identified. In addition to mere identifying a tag may incorporate or it may at least be functionally connected to one or more sensors providing measurement data to be passed forward by the communication means of the tag. A device called an RFID reader comprises an RF transceiver, or at least a receiver in the case of active tags described hereinafter, which can be used to remotely acquire the data from the tag whereby the physical distance between the reader and the tag may vary from few centimeters to hundreds of meters depending on the nature of the tag and prevailing conditions such as the presence of obstacles or interference in the radio path.
Passive tags include a transmit circuit that powers up upon absorbing radiated energy from a reader device so as to transmit the ID and optional other information to the reader via a local antenna, meanwhile active tags include, or are at least connected to, a power source of their own, which provides necessary energy for the internal chip(s) and data transmission. Such a power source may be a disposable or rechargeable battery, for example. There are also so-called semi-passive tags that indeed have a power supply for limited use but also received radiation is utilized for powering up local functionalities. Active tags may have a range of hundreds of meters whereas passive tags are limited to much shorter communication distances. In this patent application mainly active tags are discussed, but the invention is not restricted into active tags or any particular form of providing operating energy for the tag.
The tags may have processing and memory means for processing and storing instructions and other data. The tags may also have data transfer means including an antenna for sending data to external devices such as readers. The various means may be integrated in one or more chips, for example. In addition the tags may incorporate or be at least functionally connectable to one or more sensors that are configured to provide the aforementioned measurement data forward, if necessary.
RFID technology is often used in different product or human/animal tracking and/or control applications. E.g. in logistic applications the tags attached to moveable goods facilitate real-time monitoring of product status and location while the goods are being relocated or during storage. RFID tags shall preferably be light, small-sized, affordable, durable, and versatile. Such requirements are not particularly easy to implement in a single generic product, and the product development in the field has diverged into multiple directions depending on each application dictating its own requirements and preferences.
There are several types of wireless RF communications that are available for data transfer between RFID tags and other parts of the system. Especially, wireless data transfer formats have been developed for short range communications between computers and other data processing and communications equipment. In such applications it is often necessary to transfer large amounts of data, and the data transfer rate is required to be high. Also, the response delays in any such communications are required to be very short. However, such requirements also mean that the RF transceiver parts of the devices have high current consumption. This is not a problem in data processing equipment if they can be equipped with high capacity batteries or they can use mains voltage. In RFID applications, however, current consumption is an important issue, and therefore the high rate communication channels are generally not suitable for communication between RFID tags and a master device.
There also exist frequency channels wherein the form of the data transfer is not restricted, and which are used for slow rate communications. Examples of such channels are located in e.g. 433.92 MHz or 868-915 MHz frequency ranges. In prior art wireless sensor systems, the master device controls the usage of the channel. The master device may thus initiate the data transfer with one RFID device at a time. It is also possible that the master device allocates certain time slots for each RFID device of the system. The RFID devices can then listen to the timing signals broadcasted by the master device, and use their allocated time slots for sending data to the master device. However, this prior art solution also includes a problem concerning current consumption of the RFID devices. If the master device initiates the data transfer with an RFID device, the RFID device must continuously receive and monitor the RF channel in order receive the data transmission requests from the master device. On the other hand, if the master device allocates certain time slots for the RFID devices, the RFID devices must listen to the timing signals from the master device and wait for their turn to be able to transmit data. So also in this case the RFID transceiver must be powered most of the time.
It is desirable that the RFID devices can be installed to e.g. mobile targets, and that the RFID devices remain operable for long periods without a need for servicing. It is also desirable that the RFID devices are small in size and inexpensive to manufacture. However, an RF transceiver has generally an essential consumption of power in its active state, and therefore, long operation times would require batteries of large size and high cost in the RFID devices.